The present invention relates to an image forming apparatus of the type using a two-ingredient type developer and transferring a toner image formed on an image carrier by the developer to a recording medium and, more particularly, to a cleaning device for removing toner left on the image carrier after image transfer.
An electrophotographic copier, laser printer, facsimile apparatus or similar image forming apparatus has an image carrier implemented by a photoconductor. The image carrier is uniformly charged and then exposed or optically scanned to electrostatically form a latent image thereon. The latent image is developed by a developer to turn out a toner image. The toner image is transferred to a paper sheet or similar recording medium to produce a copy. The developer is either a single-ingredient type developer, i.e., toner or a two-ingredient type developer which is a mixture of magnetic carrier and toner.
Ideally, the toner of the developer should be entirely transferred to a sheet. In practice, however, a part of the toner remains on the image carrier after image transfer. It has been customary with an image forming apparatus to remove the residual toner from the image carrier by use of a cleaning device. The cleaning device has a blade or a brush contacting the surface of the image carrier and for scraping off the toner mechanically. Usually, the toner removed from the image carrier is collected in a tank and then simply disposed of. However, it is desirable to recycle the waste toner in order to save limited resources.
In light of the above, there has been proposed an image forming apparatus capable of removing residual toner from an image carrier at a cleaning step, again depositing, or redepositing, the toner on the image carrier, and then collecting the toner conveyed by the image carrier in a developing device. Examples of this type of apparatus are disclosed in (1) Japanese Patent Publication No. 61-30274, (2) Japanese Patent Laid-Open Publication No. 6-51672, and (3) Japanese Patent Laid-Open Publication No. 5-61388.
In principle, the above apparatuses (1) and (2) each electrostatically attracts the toner remaining on the image carrier at a cleaning step, and then redeposits it on the image carrier. As the image carrier conveys the toner to the developing device, the toner is collected by the developing device. However, these apparatuses have various problems, as follows.
Generally, at the image transferring step, the toner to be transferred from the image carrier to a sheet is partly inverted in charge due to a transfer bias, i.e., it is charged partly to the positive polarity and partly to the negative polarity. Hence, at the cleaning step, only the toner of one polarity is electrostatically removed from the image carrier while the toner of the other polarity is left on the image carrier. Further, the apparatus (1) uses a lamp and corona discharge for implementing a discharging step which also joins in cleaning. This kind of discharging scheme, however, complicates the structure and produces ozone.
The apparatus (3) uniformizes the toner to be attracted in the cleaning step to the same polarity by frictional charging relying on a fur brush. However, because the probability that the fur brush contacts the toner is low, it is difficult to set up a frictional force intense enough to invert the polarity of the toner.
This again prevents the entire residual toner from being collected. In addition, the apparatus (3) needs not only the fur brush but also an extra member for collecting the toner from the fur brush, resulting in a complicated structure.
The residual toner will be surely attracted at the cleaning step if a uniform charge is deposited on the toner remaining on the image carrier, and if a bias opposite in polarity to the charge is applied. However, it is likely that the residual toner is partly inverted in polarity due to the influence of the polarity of the bias. Particularly, at the time of image transfer using a bias opposite in polarity to the bias for development, the charge of the toner sometimes adapts itself to the polarity of the transfer bias and consequently has the same polarity as the bias for attraction. Hence, even if the bias for the redeposition of the toner is provided with the same polarity as the charge of the toner and if a high voltage necessary for the transfer of the toner to the image carrier due to repulsion is applied, the toner opposite in polarity to the above bias due to the change in polarity is not transferred to the image carrier, but it remains on the cleaning member. As a result, the toner collection efficiency in the developing device is lowered. Moreover, in order that the toner collected by the cleaning member and including the toner whose polarity has changed may be redeposited on the image carrier by repulsion, there is needed a particular charging system for regularizing all the toner to the same polarity.
Frictional charging is a common implementation for charging the toner deposited on the cleaning member to the same polarity. For frictional charging, the image carrier and cleaning member are held in contact with each other to form a nip therebetween. The image carrier and cleaning member are each moved at a particular speed at the nip, thereby charging the toner by friction. However, frictional charging is not practicable without resorting to a great nip and, therefore, without increasing the size of the cleaning member. This increases the overall size of the apparatus. Further, a great nip reduces the area of the cleaning member available for attracting the toner and thereby obstructs the collection of the toner. In addition, when the cleaning member is implemented as a roller, an increase in the nip results in a decrease in the attraction area of the cleaning member, as measured in the circumferential direction. As a result, the probability that the toner retained on the cleaning member approaches the nip is high. Because the bias for image transfer is far higher than the bias used for the cleaning member to attract the toner, the toner on the cleaning member and adjoining the image carrier is reversely transferred to the image area of the image carrier due to the difference between the biases.
On the reverse transfer of the toner to the image area of the image carrier, the amount of toner to be frictionally charged at the nip increases to an unusual degree and increases the load on frictional charging, thereby obstructing the uniform polarity conversion. It is, therefore, difficult for the cleaning member to collect all the residual toner from the image carrier.
Assume that the cleaning member, whether it be moved in the same direction as or the opposite direction to the image carrier, retains the collected toner in an area less than its length as measured in the direction of movement. Then, only the limited area of the cleaning member is always used to attract and redeposit the toner. For example, when the cleaning member is implemented as a roller, only a part thereof repeatedly contacts the image carrier for the attraction and redeposition. As a result, the cleaning member is caused to locally wear and lower its cleaning characteristic. In this condition, the cleaning member is apt to fail to collect all the toner from the image carrier.
The image carrier is charged due to the influence of biases for image transfer and cleaning, depending on the material thereof. While this kind of charge deposited on the image carrier is usually dissipated before the next image formation, it cannot be done so, depending on the material of the image carrier. To dissipate the undesirable charge, use is made of a lamp in consideration of the photoconductive layer provided on the image carrier. However, the charge dissipation using a lamp is effective only if the charge is of negative polarity. Specifically, this kind of charge dissipation scheme is not applicable to a photoconductive layer formed of OPC (Organic Photo Conductor) or similar organic substance, because the organic substance sometimes allows a charge of positive polarity to remain thereon. Should the lamp scheme be practiced in combination with an organic photoconductive layer, the surface potential to be set by the next charging step would be irregular and change the potential distribution of a latent image, thereby adversely effecting the density of the resulting image.
For the transfer of the toner image from the image carrier to a sheet, use is made of a conductive member capable of contacting the image carrier. While the conductive member conveys the sheet in cooperation with the image carrier, a transfer bias is applied to the member in order to electrostatically transfer the toner image to the sheet. Usually, the transfer bias is stopped as soon as the conductive member fully conveys the sheet. However, when the trailing edge of the sheet is about to move away from the conductive member, the transfer bias influences the residual toner on the image carrier because the conductive member is positioned close to the image carrier. The conductive member is, therefore, apt to attract the toner from the image carrier. This part of the toner is transferred from the conductive member to the rear of the next sheet and smears it.
The collection of the residual toner from the image carrier is also performed for the initialization purpose. For example, just after the start-up of the apparatus or at the time of warm-up, cleaning is executed for initialization. Further, after the stop of operation of the apparatus due to a sheet jam or similar trouble, the image carrier is again rotated for initialization. As to the initialization to occur after a trouble, a great amount of toner, including toner to be transferred to a sheet, exists on the image carrier. The amount is sometimes too great for the cleaning member to remove by attraction, so that the toner sometimes partly remains on the image carrier. Then, when use is made of a charging device of the type contacting the image carrier, the toner penetrates between the charging device and the image carrier and makes it impossible for the charging device to perform uniform charging. As a result, white stripes appear on the image carrier and render the resulting image defective.
In order to obviate the above drawback, the voltage may be controlled in such a manner as to intensify the electric field to act on the cleaning member, as in the previously stated apparatus (2). This, however, brings about another problem that the photoconductive layer of the image carrier suffers from noticeable electrostatic fatigue and reduces the life of the image carrier. This is partly because the direction of the electric field is switched over in matching relation to the intensities of electric fields assigned to the attraction and redeposition of the toner, and partly because a relatively high potential for discharge is repeatedly applied.
Assume the cleaning member is constantly held in contact with the image carrier. Then, the cleaning member contacts the same part of the surface of the image carrier every time the image carrier is brought to a stop. When the cleaning member is implemented as a roller, the surface of the image carrier partly undergoes transformation because the components of the roller separate or because they chemically react with the photoconductive layer of the image carrier. This causes white stripes to appear in an image to be produced when the image carrier is started up later.
Furthermore, if the toner exists at the nip between the image carrier and the cleaning roller when the image carrier is brought to a stop, it is often degenerated. Particularly, in a hot and humid environment, the toner is likely to solidify and cause the cleaning roller to loose elasticity at the nip. This degrades the toner removing ability of the cleaning roller.
Assume that the charging member for executing the charging step subsequent to the cleaning step is of the type injecting charge in the image carrier in contact therewith. Then, the defective cleaning described above causes the residual toner to deposit on the charging member and prevent it from performing uniform charging.
On the other hand, the apparatuses (1) and (2) do not take account of problems particular to a developing system which develops a latent image by depositing toner of the same polarity as the charge of the image carrier in the exposed portion of the image carrier, i.e., so-called reverse development. Specifically, in a reverse developing system, a charger is usually held operative at all times because toner deposits in non-charged portions. However, when charging and discharging are effected over the toner which has been redeposited on the image carrier between images as usual, the amount of charge deposited on the toner increases with the result that the potential of the image carrier is caused to differ from the portions where the toner is present to the portions where it is absent. The irregular potential distribution on the image carrier, coupled with the increased amount of charge, makes it difficult to remove the toner from the image carrier and thereby makes the toner collection at the developing device defective.